Photoinduced Halogen-Atom Transfer (XAT) by N-heterocyclic carbene boryl radicals for C(sp3)−C(sp3) bond formation

In the realm of radical chemistry, halogen-atom transfer (XAT) is emerging as a powerful activation strategy in synthetic endeavors to engage otherwise recalcitrant organic halides. Although toxic tin radicals have been in the spotlight as halo-gen abstractors for decades, the era of photocatalysis has brought to light more sustainable options. Herein, we present a comprehensive study on the use of ligated boryl radicals to enable C(sp3)−C(sp3) bond forming reactions via XAT from alkyl halides. Two strategies have been developed using either direct light activation with near UV or photoredox conditions with visible light; both pivoting on N-heterocyclic carbene (NHC) boryl radicals. The present XAT strategy is very mild and accommodates a broad scope of alkyl halides, including medicinally-relevant compounds and biologically-active molecules. The key role of NHC boryl radicals in the operative reaction mechanism has been elucidated through a combination of ex-perimental, spectroscopic and computational studies.